Eerie audio recordings captured on Mars have revealed, for the first time, that there are two speeds of sound on the red planet, among other trippy insights about how acoustic waves travel on an alien world.
NASA’s Perseverance rover, which landed on Mars in February 2021, is the first mission ever to capture recordings of the Martian soundscape. Though some of Perseverance’s tracks have already dropped, scientists led by Sylvestre Maurice, an astrophysicist at the University of Toulouse, now “present the first characterization of Mars’ acoustic environment and pressure fluctuations in the audible range,” featuring “pressure variations down to 1,000 times smaller scales than ever observed before,” according to a study published on Friday in Nature.
“Prior to the Perseverance rover landing, the acoustic environment of Mars was unknown,” the team said in the study. “The recording of sounds offers the unique opportunity to study the atmosphere as the main natural source of sound, and as the propagation medium for acoustic waves.”
Two previous NASA Mars missions, the Mars Polar Lander and the Phoenix lander, carried microphones to the red planet, but these early attempts to record Martian sounds were dashed when Phoenix’s audio instruments malfunctioned and the Mars Polar Lander crashed on the planet’s south pole.
In order to finally capture Mars’ elusive noises, Perseverance was outfitted with two microphones that are paired with its cameras. One is embedded in the SuperCam, which is the instrument suite on the head of the rover’s mast, while the other is part of the Entry, Descent, and Landing (EDL) Camera, which was placed on the rover’s port frame in order to capture Perseverance’s daredevil landing on Mars.
The SuperCam has picked up the clicking sounds of Perseverance’s laser instrument as it shoots nearby rocks, as well as the whir of spinning rotors on Ingenuity, a small helicopter that is still conducting flights on Mars a year after it became the first powered aircraft ever to fly on another planet. The EDL microphone has captured the sounds of the Martian wind, and the noise made by Perseverance’s Gaseous Dust Removal Tool as it blows detritus off of rocks so that the rover can study them.
The new audio confirms that the speed of sound is slower on Mars than on Earth, a result that was expected since the motion of acoustic waves is modulated by the density of substances they occupy. For instance, here on Earth, the speed of sound is faster in the dense medium of water than it is in the air.
On Mars, the atmosphere is 100 times thinner than on Earth, so it makes sense that sound travels at roughly 550 miles per hour on the red planet, compared to a speed of 767 miles per hour on Earth. However, the team also discovered that there are at least two Martian speeds of sound that vary by pitch, or frequency, of the sound waves, a phenomenon that has only been observed on Mars.
Low-pitched sounds on the planet move at roughly 537 miles per hour and high-pitched sounds travel at 559 miles per hour. Maurice and his colleagues suspect that this strange delay in low notes is caused by the low pressure and thermal turbulence of Martian surface air, which combine to help high-frequency acoustic waves propagate more quickly.
“Due to the unique properties of the carbon dioxide molecules at low pressure, Mars is the only terrestrial-planet atmosphere in the Solar System experiencing a change in speed of sound right in the middle of the audible bandwidth (20 Hz – 20 kHz),” said the team in a study presented at the 53rd Lunar and Planetary Science Conference last month.
“It may induce a unique listening experience on Mars with an early arrival of high-pitched sounds compared to bass,” the researchers added.
In other words, if you were standing on Mars and were able to hear music playing nearby, the high notes would reach you before the low notes. You would need to be very close to this speculative Martian band to hear anything at all, as Maurice and his colleagues also found that sound begins to drop off at just 26 feet from a source, as Martian air is such an inefficient vector for acoustic waves. This makes Mars an uncannily quiet world.
The new findings not only bring Martian sounds to our Earthling ears, they also offer a new means to study the atmospheres of alien worlds and even assess the condition of our robots as they explore extraterrestrial surfaces.
“Sound is a new, rich source of information on Mars,” the team said in the study. “Thanks to sensors measuring a few millimeters in diameter only, turbulence-induced noise and artificial sources have been recorded.”
“These results establish a ground truth for modeling of acoustic processes, which is critical for studies in atmospheres like Mars and Venus,” the researchers concluded.